1. Field of the Invention
The invention relates to a security element, preferably for documents of value, which at least has one area with a diffraction structure, which under specific viewing conditions reconstructs a diffractive image, wherein the area has subareas, which do not take part in the reconstruction of the diffractive image, and which represent a recognizable information. The invention further relates to a data carrier with such a security element, an embossing cylinder and a method for producing such a security element or document of value.
2. Description of the Background Art
Documents, in particular documents of value and identification documents, but also commercial goods and consumer durables or their packagings for the purpose of preventing forgeries and checking their authenticity often are equipped with special security features. Due to the easy availability and high quality of reproductions, which can be produced with modern color copying machines or by high-resolution scanners and color laser printers even by poorly trained staff, there is a steady demand for improving the forgery-proofness of the respective security features. For this purpose optically variable security elements proved to be good, which under different viewing directions produce different optical impressions. Such security elements, for example, have optically diffraction structures, which under different viewing angles reconstruct different images. Such effects cannot be reproduced with the usual and widespread copying techniques and printing techniques.
EP 1 197 350 A2 describes a special embodiment of such a diffractive security element for documents of value. It is a so-called two-channel hologram, which under different viewing directions reconstructs different holographic images. Each holographically reproduced image, which is visually recognizable under only one viewing direction, corresponds to one channel. To the individual channels are associated different surface areas of the hologram. The area of the hologram corresponding to a channel can be divided into a plurality of partial areas. These partial areas are formed, for example, as narrow strips. The strips belonging to different channels preferably are disposed alternatingly. Although the partial areas of a diffractive image, which belong together, are spaced apart from each other, under the specific defined viewing angle they produce a joint diffraction image. In the strip-shaped partial areas, preferably with the help of a laser, subareas are changed such that they no longer take part in the image reconstruction and represent a first or second individual information against the holographic background.
As to obtain this effect, it is necessary to very exactly incorporate the pieces of information produced with the laser into the partial areas or strips each associated with a channel. I.e., during the laser marking process at first it has to be determined to which channel the strip to be marked is associated. This is effected by checking the direction, in which incident light is diffracted and thus a holographic image is reconstructed. This check is effected, for example, by focused LEDs as light sources and optical detectors, for example a camera for determining the pertinent viewing angle. Such a method is very elaborate, in particular the efforts and the required precision will increase with the fineness of the partial areas, into which the individual partial channels are spatially divided. Hence this proceeding is hardly suitable for a rationalized large scale production method.